1. Field of the Invention
The present invention relates generally to artificial lungs and, more particularly, to a new configuration intravascular membrane lung which, after percutaneous insertion, will be capable of exchanging the entire basal oxygen consumption and carbon dioxide production of an adult man or woman.
2. Description of the Prior Art
Intravascular membrane lungs have notable benefits. They do not require blood pumps, nor lung resection. In addition, the intravascular membrane lung has the advantage that the skin and circulation need only be violated at one location for its insertion at a peripheral site. This may lessen the risk of infection. However, known devices also have significant shortcomings. Unfortunately, the largest model of the most advanced current design, the intravenous oxygenator (IVOX), disclosed in U.S. Pat. No. 4,583,969 to Mortensen, can at best exchange only approximately 40% of basal metabolic needs of the adult patient. Most current designs of intravascular membrane lungs cannot be inserted percutaneously, and require cutdown on the vessel prior to insertion. Any device which is to have widespread use must be capable of rapid insertion using something similar to the well known Seldinger technique. Membrane lungs mounted paracorporeally outside the chest wall, with or without a blood pump, is actually extracorporeal membrane oxygenation with a special cannulation site. Also, a device with a single insertion site has little effect on the turning of a patient for chest physical therapy as would a paracorporeal mounted lung with two cannulae protruding from the chest.
The physical/chemical properties of the oxygen dissociation curve presented in FIG. 1 shows the limit of the amount of oxygen which can be transferred into a given blood flow stream by an intravascular lung. Even if an intravascular lung had no convective or diffusion limitations to oxygen transfer, the maximum oxygen transfer would still be limited by the blood flow rate across the device and the oxygen saturation of the input blood. This occurs because the oxygen saturation curve constrains the oxygen content of the blood exiting the device. Hence, the maximum oxygen transport is limited to the product of the difference between 100% flow rate over the device, and the oxygen carrying capacity of the blood. A gas exchanger such as the IVOX of the Mortensen patent which primarily processes inferior vena caval blood, or approximately half of the cardiac output, can only expect at best to transfer a maximum of 40-50% of basal oxygen requirements.
On the other hand, if the gas exchange surface or the membrane lung were placed not only in the inferior vena cava, but also extended into the right ventricle and the pulmonary artery, the low saturation blood returning from the coronary sinus as well as that of the superior vena cava could be oxygenated. Also, the opening and closure of the tricuspid valve, the contraction of the right ventricle and the opening and closing of the pulmonic valve produce intravascular secondary blood flows. This may reduce the resistance to oxygen transfer of the blood boundary layer adjacent to the membrane lung gas exchange surface. In addition, other intravascular lung designs have shown that it is difficult to achieve a closer packing of fibers in the inferior vena cava than the current IVOX has without interfering with venous return.
In the Mortensen, or IVOX, device, noted above, hollow fibers of 25 to 65 centimeters in length are mounted and extend between two spaced apart manifolds. As mentioned, the device is intended to be placed only in the vena cava. Oxygen is passed through the hollow fibers, and gas exchange occurs through the permeable membrane with the blood of the vena cava. In an initial design, gas entered through a cannula in the femoral vein and exited through a cannula in the right internal jugular vein. In a later design, a concentric double catheter allows gas flow to occur through a single cannula. The device is inserted by surgical isolation of the access vessel and advanced into the vena cava with only the tip of the device lying in the superior vena cava. In this position, most of the surface area of the gas exchange membrane is exposed to blood returning to the right atrium via the vena cava. The gas exhaust limb is open to the atmosphere and the gas supply pressure is kept at less than 15 mm Hg (gauge). Gas flow through the IVOX is provided by supplying gas at atmospheric pressure to the inlet manifold and drawing a partial vacuum at the exhaust manifold. Gas flows up to 3 liters/minute have been obtained. This method of obtaining gas flow has been utilized to reduce the risk of positive pressure within the microporous hollow fibers forcing gas bubbles into the vena cava.
Variations on the Mortensen design are disclosed in U.S. Pat. Nos. 4,986,809 and 4,911,689 to Hattler and U.S. Pat. No. 4,850,958 to Berry et al. In the Hattler oxygenator, a plurality of hollow, gas permeable fibers extend from a Y-shaped tubular connector either to a ring or to a tip end, then, in loops, return to the connector. This arrangement is percutanaeously inserted into a vein and, once in place, occupies the superior vena cava, inferior vena cava, right atrium, or some combination of these areas in the patient. The patent explains that the fiber loops can be crimped and/or twisted into a helical arrangement to enhance gas exchange. The Berry et al. apparatus includes a metal rod for structural support of the gas permeable tubes and that apparatus is intended for placement within the venae cavae of a patient.
Also known are lung assist devices such as that disclosed in U.S. Pat. No. 5,037,383 to Vaslef et al. The Vaslef et al device is comprised of short subunits of shorter looped hollow fibers with several subunits placed along a central gas supply and exhaust line. These have been tested in a cylindrical blood flow channel to determine gas exchange parameters and resistance to blood flow. As reported in Vaslef, S. N.; Mockros, L. F.; Anderson, R. W.: "Development of an Intravascular Lung Assist Device"; Transactions of the American Society of Artificial Internal Organs; Vol. XXXV:660-664, 1989, up to 100 cc of CO.sub.2 and O.sub.2 gas exchange were possible with devices with a greater number of fibers but with unacceptable pressure drops across the device of up to 100 mm Hg at 4.7 liters/min.
Still another variation of known oxygenators is that disclosed in U.S. Pat. No. 4,631,053 to Taheri which discloses a disposable device for insertion into the inferior vena cava of a patient. It includes a hollow tubular gas permeable membrane having numerous side branches said in the patent to resemble pine needles on a pine branch. The membrane is mounted on a support wire and is surrounded by a sheath through which blood can flow. The sheath is also secured to the support wire. It is unclear from a study of this patent as to whether the gas permeable membrane or the pine needles themselves provide the major portion of gas exchange. There is no description as to how to optimize either the shape, length or number of fibers to provide gas exchange. Also, the device is located in the lower part of the inferior vena cava and, at best, could only oxygenate and decarbonate blood returning from the lower extremities.
A major problem posed by known artificial lungs using microporous membranes as the gas exchange surface is that they can lose their ability to transfer oxygen and carbon dioxide in as little as four to six hours after the beginning of extracorporeal circulation. This deterioration has been attributed to condensation of water in the gas phase or the transudation of plasma from the blood phase across the microporous membrane phase. In Mottaghy, K.; Oedekoven, B.; Starmans, H.; Muller, B.; Kashefi, A.; Hoffman, B. and Bohm, S.: "Technical Aspects of Plasma Leakage Prevention in Microporous Capillary Membrane Oxygenators"; Transactions of the American Society of Artificial Internal Organs; Vol. XXXV:640-643, 1989, Mottaghy et al. reported a method for prolonging the use of microporous hollow fibers by heating of the gas flushing the membrane lung. They postulated that the temperature of the gas passing through the hollow fibers has a significant effect on the cooling and condensation of liquid passing through the micropores. In normal operation the gas is cooler than the blood and thereby cools the water vapor within the gas phase causing condensation and filling of the micropores. The condensed water was further postulated to pull plasma across the microporous membrane by capillary action. By heating the gas to a temperature of about 2.degree. C. greater than blood temperature, use of this type of membrane was extended to a duration of five days without any decrement of gas exchange. This represents a significant step in the quest for developing a successful artificial lung.
Hollow fibers of microporous polypropylene generally of the type disclosed in U.S. Pat. No. 4,770,852 to Takahara et al. have been used as the gas exchange surface in membrane lung gas exchangers designed for short term use during cardiopulmonary bypass for cardiac surgery. These devices have shown excellent gas exchange with little hemolysis or formed element damage. Importantly, the raw microporous surface has a maximal gas exchange which is decreased by coating it with any continuous polymer such as silicone. Recent studies have shown that microporous membranes will not degrade their performance for at least a week if gas heated above the body temperature is used to ventilate the fibers. Finally, polypropylene is capable of covalent heparin bonding via the CARMEDA.RTM. Process, a proprietary process licensed to and commercialized by the Cardiopulmonary Division of Medtronic, Inc., of Anaheim, Calif.
As evidenced by the patents, noted above, particularly those to Berry et al., Hatter, Mortensen, and to Vaslef et al, present intravascular lungs which use hollow fibers for gas exchange have these fibers tethered at both ends of their gas conduit catheter. Thus, the gas flushing the catheter sweeps through the lumen of the fibers and convects oxygen to the wall of the fiber for diffusion out while the carbon dioxide, which has diffused in, is convected away. This method of mounting the fibers results in the direction of much of the blood flow being in parallel with the axes of the fibers. In contrast, in the device of the invention, the fibers are tethered at only one end to a catheter while the other ends of the fibers are sealed and float freely generally transversely of the blood stream. In this manner, blood flow occurs transversely of, or across, the axes of the fibers floating in the blood stream. This cross flow arrangement of fibers and blood flow optimizes oxygen transfer. By use of fibers tethered only at one end, diffusion of the oxygen and carbon dioxide along each hollow fiber from the fiber wall to the gas flushing the central catheter becomes a major process in mass transfer which may be augmented by secondary gas flows set up by high frequency oscillations of the supply gas pressure. Such high frequency oscillators are in common use for augmenting gas exchange in the natural lung. The exact mechanism of augmented secondary flow is unknown. However, having a gas with compressable properties will probably allow an augmentation of diffusion down the axis of the fiber.